The present invention relates in general to semiconductor devices, and more particularly to a method and structure for making a topside contact to a semiconductor substrate.
In some semiconductor devices (e.g., vertically-conducting power devices), the substrate forms a bottom terminal of the device, and various techniques have been used to form a low resistance contact to the bottom terminal. FIG. 1A shows a cross-sectional view of a conventional device structure with a backside contact. As shown, a N− region 101 is formed over a N+ substrate region 102. A conductive interconnect layer 103 formed at the bottom of the substrate is used as a backside contact. For certain applications it may be desirable to contact the substrate from the topside of the device. FIGS. 1B-1C show cross-section views illustrating two conventional techniques for contacting the bottom terminal of a device through the topside.
In FIG. 1B, a heavily doped diffused region 105 extends through N− region 101 to reach N+ substrate region 102. A conductive interconnect layer 107 is formed over diffused region 105, which together with diffused region 105 forms a topside contact to N+ substrate region 102. In FIG. 1C, a deep trench 108 is formed through N− region 101 to reach N+ substrate region 102. Then a conductive material 109 is used to fill the trench, thus forming a topside contact to N+ substrate region 102.
Even though these conventional techniques have been used for making topside contact to the bottom terminal, there are limitations associated with these techniques. For example, diffused region 105 in FIG. 1B requires a high temperature drive-in process after a diffusion or implant step. This leads to wide lateral out-diffusion and high thermal budget. In FIG. 1C, the process of making a deep trench and then filling it with a conductive material is often complicated. If polysilicon is used to fill the trench, it is often difficult to obtain highly doped polysilicon to form a low resistivity topside contact.
Thus, there is a need for a technique whereby a low resistance topside contact is made to the substrate while maintaining a simple manufacturing process.